Polymer blends represent an important class of polymeric materials, accounting for 20% of the U.S. synthetic resin market in 1985. Although the majority of commercially important polymer blends are thermodynamically immiscible heterogeneous blends, considerable interest exists for producing miscible homogeneous blends, because such blends achieve properties intermediate to those of their constituent polymers. Commercial miscible blends are at present mainly used for their mechanical and thermal properties, but immense potential exists for the development of polymer blends with specially tailored optical, surface, barrier or biodegradation properties. In spite of much synthetic research effort, miscible polymer pairs are rare, because the high molecular weight of polymers provides little entropic driving force for miscability. Favorable enthalpic contributions, which lead to miscability, are possible only when specific interactions are present between moieties in the different polymers.
In an attempt to bypass the specific interactions requirement, researchers have studied potential techniques for producing nonequilibrium single-phase blends of thermodynamically immiscible polymers. These techniques typically involve the preparation of a dilute ternary solution of two polymers in a common liquid solvent, followed by rapid quenching of the blend into the solid state via coagulation, solvent evaporation or freeze-drying. These diffusion-governed processes are slow, as compared with incipient polymer phase separation in liquid solutions, so it is not surprising that some degree of phase separation occurs, e.g., micron-sized domains are formed or extremely broad glass transition temperatures are noted.
In light of the fact that many polymers (e.g., all glassy polymers) enjoy commercial use in a thermodynamic state far removed from equilibrium, it is short-sighted to limit the search for intermediate-property polymer blends to systems that are thermodynamically miscible. For example, rapid coagulation or quenching of polymer mixtures can result in a frozen-in single phase morphology, such as that observed in molecular composites. However, coagulation and quenching techniques are governed by solvent and heat diffusion, respectively, which are typically slow processes in high molecular weight systems, and can cause an undesirable skin/core morphology.
To summarize, attempts by others to produce a homogeneous mixture of immiscible polymers or copolymers have involved processes such as solvent evaporation, coagulation, rapid freezing or freeze-drying. Each of these techniques is governed by a diffusive process, either of heat or of mass. Particularly in polymercontaining materials, diffusive processes are typically slow. The present invention produces a homogeneous mixture of immiscible polymers by density reduction of a solution whose solvent is above its critical point. The time scale of this process is governed by convection (i.e., bulk flow), not diffusion, and the characteristic convective velocity for flow of a compressible fluid such as a supercritical fluid is the speed of sound, which is quite rapid.
Producing a homogeneous mixture of immiscible polymers is of practical interest for several reasons. The vast majority of pairs of polymers are immiscible, so they separate into two different phases when they are mixed. Because of the phase separation, the immiscible mixtures are opaque and have poor mechanical properties, even to the point of having little mechanical integrity. In homogeneous miscible blends, however, transparency and good mechanical properties can be achieved. The properties are usually intermediate to the two components, so producing homogeneous mixtures of polymers offers the opportunity to achieve adjustable properties without the need for synthesizing new polymer materials.
Besides the usefulness of producing stable homogeneous polymer mixtures for the sake of their properties, it may be desirable to produce unstable homogeneous polymer mixtures that will revert in time to their thermodynamically stable, immiscible form. Since the rate of phase separation will be dependent on the environmental conditions, these materials could be used as environmental indicators, or as environmentally degradable materials.
According to the present invention, a homogeneous single-phase blend of otherwise immiscible polymers is prepared via a precipitation technique which is not limited by slow diffusion processes. The solid binary blend is precipitated by rapid pressure (density) reduction from a homogeneous ternary solution of the two polymers in a supercritical fluid (SCF) solvent. Single-phase polymer blends are produced thereby.